Who were Syed Rizwan Farook and Tashfeen Malik?

(CNN)While police say they know who massacred 14 people Wednesday at a holiday party in San Bernardino, California, the killers' motives remain a mystery.

Syed Rizwan Farook and Tashfeen Malik committed the deadliest U.S. mass shooting since Sandy Hook in December 2012 before being killed by police, authorities say.

Here is what we know about them:

Husband and wife

Farook, 28, and Malik, 27, had been married for two years, according to Hussam Ayloush, head of the Greater Los Angeles chapter of the Council on American-Islamic Relations, a Muslim advocacy group.

The couple left their 6-month-old girl with Farook's grandmother Wednesday and said they had a doctor's appointment.

The grandmother became concerned when she saw reports of the shooting and couldn't reach Farook.

Farook was environmental health specialist for county

Farook, an American citizen, was an environmental health specialist with the San Bernardino County health department, which was hosting the holiday party at the Inland Regional Center where the attack took place.

He had been there five years. Records list a man with his name and title who worked at the agency and made $53,000 in 2013.

It's important to note Farook's middle name -- Rizwan. Because his older brother is also named Syed Farook but has a different middle name.

The attackers were "dressed in dark, kind of tactical gear," San Bernardino police Chief Jarrod Burguan said. They carried rifles and semi-automatic handguns.

Two handguns recovered have been traced back to Farook. He purchased them legally three or four years ago, an official said. Two rifles were purchased by someone else, possibly a former roommate, also legally three or four years ago. Officials said they don't think that person had anything to do with the shooting.

"These were people that came prepared," Burguan said. "There had to have been some degree of planning that went into this."

A bag believed to belong to the shooters was found inside the conference room. Inside, investigators found three rudimentary explosive devices packed with black powder and rigged to a remote controlled car. The remote for the car was found inside the SUV where Farook and Malik were later killed, a law enforcement official said.

The pair planned to use the remote to detonate the explosives from a distance, the official said. Either it didn't work because of distance or they didn't do it. Officials later rendered the explosive devices safe.

After carrying out the attack at Inland Regional -- where they killed 14 people and injured at least 17 -- Farook and Malik led police on a chase. Farook fired at officers while Malik drove, officials said. They died in the shootout, which involved 21 officers.